1. Field of the Invention
The invention pertains to a method and apparatus for uniformly ablating a substrate, and more specifically, for performing photo refractive keratectomy on the eye.
2. Description of the Related Art
It is known to ablate tissue by means of laser beams for sculpting the cornea (photo-ablation). For this purpose, excimer lasers are preferably used whose wavelength are about 200 nm or more. FIG. 1 shows a known arrangement in which a laser beam of an excimer laser 10 is directed through a beam homogenizer 12 and an iris diaphragm 14 and subsequently by a mirror 18 onto the cornea of an eye 2. The size of the iris diaphragm is adjustable by means of a control device 16. Thus, the size of the spot formed on the cornea by the laser beam can be adjusted. By varying the surface area and shape of the spot and the number of laser pulses for certain sections of the surface of the cornea, the form of the cornea can be modified and an ametropia of the eye can be corrected.
FIG. 2, for example, shows how the shape of the cornea, which originally has a radius R.sub.1, is flattened in a circular area having a diameter D by a laser treatment in such a way that subsequently an effective radius R.sub.2 is achieved. In the area of the optical axis 5 of the eye 2, the ablated layer has a thickness A.
For reasons as yet unknown, the tissue is often not uniformly removed. That is, after a treatment with the known apparatus the shape of the cornea deviates from the desired shape. As schematically shown in FIG. 3, which is an enlarged view of the area marked with a circle in FIG. 2, less tissue is excised in the area of the optical axis than at the edge. Experiments have resulted in a deviation x in the order of magnitude of 5 to 10 .mu.m, measured in an area having a diameter d of 2 to 3 mm and being symmetrical to the optical axis 5. This so-called "central island" problem has so far been obviated by pre-and after-treating the central area in such a way that this elevation has been ablated by means of laser pulses while correspondingly varying the adjustment of the iris diaphragm 14. However, the results of such an after treatment are often dissatisfying.
The uniform elimination of central islands from a treated area would therefore be greatly desirable.